I. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
II. Discussion of the Prior Art
Ion guides comprising RF only multipole rod sets such as quadrupoles, hexapoles and octopoles are well known.
Whitehouse and co-workers have disclosed in WO98/06481 and WO99/62101 an arrangement wherein a multipole rod set ion guide extends between two vacuum chambers. However, as will be appreciated by those skilled in the art, since each rod in a multipole rod set has a typical diameter of around 5 mm, and a space must be provided between opposed rods in order for there to be an ion guiding region, then the interchamber aperture when using such an arrangement is correspondingly very large (i.e. >15 mm in diameter) with a corresponding cross sectional area >150 mm2. Such large interchamber apertures drastically reduce the effectiveness of the vacuum pumps which are most effective when the interchamber orifice is as small as possible (i.e. only a few millimeters in diameter).
Gas collision cells are known which are used in tandem mass spectrometers to induce fragmentation. Such collision cells comprise an enclosed chamber arranged to be maintained at an intermediate pressure e.g. 10−3−10−1 mbar. The gas collision cell is positioned within a vacuum chamber maintained at a relatively lower pressure e.g. 10−6−10−4 mbar. The gas collision cell has a relatively small inlet differential pumping aperture through which ions enter and a relatively small outlet differential pumping aperture through which ions exit. Gas is introduced into the gas collision cell typically via a gas port. Gas which has been introduced into the collision cell will then leak out through the relatively small inlet and outlet differential pumping apertures into the vacuum chamber in which the gas collision cell is housed.
An RF ion guide upstream of the gas collision cell may be provided to transport ions towards the entrance orifice of the gas collision cell. Once ions have exited the ion guide and entered the gas collision cell they may then be transported through the gas collision cell by an RF ion guide disposed within the gas collision cell. Similarly, once the ions have exited the ion guide within the gas collision cell and have passed through the outlet differential pumping aperture they may then be transported away from the gas collision cell by means of a downstream RF ion guide. However, as ions pass from the upstream ion guide to the gas collision cell, the ions exit the upstream ion guide and are no longer radially confined as they enter the gas collision cell. Some ions will therefore be lost before the ions are received and are again radially confined by the RF ion guide disposed within the gas collision cell. Similarly, once ions leave the ion guide disposed within the gas collision cell they are no longer radially confined as they exit the gas collision cell. Some ions will therefore likewise be lost before the ions are once again radially confined within an RF ion guide disposed downstream of the gas collision cell.
It is therefore desired to provided an improved gas collision cell.